Climb a cold post outside (6)
I believe the answer is:
ascend
'climb' is the definition.
(climb can mean to ascend or go up)
'a cold post outside' is the wordplay.
'cold' becomes 'c' (eg on taps).
'post' becomes 'send' (synonyms).
'outside' is an insertion indicator.
'c' placed within 'send' is 'scend'.
'a'+'scend'='ASCEND'
(Other definitions for ascend that I've seen before include "Mount" , "Travel upwards" , "Come up" , "Gain height" , "Get higher" .)